Electroluminescent devices are opto-electonic devices where light emission is produced in response to an electrical current through the device. The physical model for EL is the radiative recombination of electrons and holes. The term light emitting diode (LED) is commonly used to describe an EL device where the current-voltage behavior is non-linear, meaning that the current through the EL device is dependent on the polarity of the voltage applied to the EL device. Both organic and inorganic materials have been used for the fabrication of LEDs. Inorganic materials such as ZnS/Sn, Ga/Bs, Ga/As have been used in semiconductor lasers, small area displays, LED lamps, etc. However, the drawbacks of inorganic materials include difficulties to process and to obtain large surface areas and efficient blue light.
Organic materials, which includes both small molecules and polymeric materials, offer several advantages over inorganic materials for LEDs, such as simpler manufacturing, low operating voltages, the possibility of producing large area and full-color displays. Conjugated polymers such as poly(phenylvinylene) (PPV) were first introduced as EL materials by Burroughes et al in 1990 (Burroughes, J. H. et al, Nature 1990, 347, 539-41). Tremendous progress has been made since then to improve the stability, efficiency, and durability of polymeric LEDs (Bernius, M. T. et al, Adv. Mater. 2000, 12, 1737). Organic LED (OLED) represents an alternative to the well-established display technologies based on cathode-ray tubes and liquid crystal displays (LCDs), especially for large area displays. OLED has been demonstrated to be brighter, thinner, lighter, and faster than LCDs. Moreover it requires less power to operate, offers higher contrast and wide viewing angle (>165 degree), and has great potential to be cheaper to manufacture, especially the polymer-based LEDs (PLED).
The OLED technology has stimulated intensive research activities across all disciplines. Currently, great efforts in materials research have been focused on efficient novel materials for full-color flexible displays. Full-color displays require three basic colors, red, green and blue, and flexible substrates require low temperature and easy processing of the organic materials. PLED devices show great promise in meeting both requirements, since the emission color can be tailored by modulation of the chemical structures and the solution processing allows for micro-patterning of the fine multicolor pixels via inkjet printing technique. However, processable, stable, and efficient blue light emitting organic materials are still highly desirable to meet the challenge. Blue light requires a wide energy band. With blue light emitting polymers as primary materials, it is possible to produce other colors by a downhill energy transfer process. For instance, a green or red EL emission can be obtained by doping a blue EL host material with a small amount of green or red luminescent material.
Furthermore, efficient EL devices require balanced charge injection and transport. The turn-on voltage depends on the energy barriers for charge injection. The mobility of the injected charges in organic polymers is low compared to that in inorganic semiconductors, which might increase the recombination time for the electrons and holes in the polymer. Moreover, most EL polymers transport preferentially either electrons or holes that leads to the recombination zone close to the electrodes and quenching of excitons. This is one of the main sources of reduced efficiency. Many research efforts have been focused on the design of polymers with improved charge injection and transporting.
It is known that imine nitrogens (C═N) are electron-deficient and are capable of transporting electrons. Oxadiazoles, pyridines, quinolines, quinoxazolines, triazines, and bithiozoles all process electron transporting properties. Electron transporting small organic molecules such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) have been dispersed in inert polymers or blended with EL polymers to improve electron transporting capability (Zhang, C. et al. Synth. Met. 1995, 72, 185; Berggren, M. et al. Adv. Mater. 1995, 7, 900). However, the incompatibility in general leads to phase separation or recrystallization especially at higher temperature during device operation. Oxadiazoles containing polymers have been intensively investigated. A number of polymers containing oxadiazole moiety have been reported by several research groups (Yang, Y. et al. Chem. Mater. 1995, 7, 1568). However, most of these polymers do not meet the requirement of low driving voltage or improved efficiency.